FIELD OF THE INVENTION
The present invention relates to image display screens which incorporate a black matrix intended to improve the contrast and the rendition of these images. Black matrices are used both in flat screens and in cathode-ray tubes. The invention provides a novel compound for forming such a black matrix, which can be used especially in plasma display panels (PDPs).
BACKGROUND OF THE INVENTION
In general, each image displayed on a PDP consists of a set of light discharge points. The light discharges occur in a gas contained between two insulating tiles, namely a front tile and a rear tile. Each discharge point is generated in a discharge cell defined by the intersection of electrodes grouped in arrays, each electrode array being supported by at least one of the tiles.
Thus a PDP has a two-dimensional matrix of cells, organized in rows and in columns, according to the geometry of the electrode arrays.
In PDPs, the emission of visible light, needed for an image to be seen by an observer, is obtained by the excitation of phosphors by ultraviolet radiation. The phosphor layers, usually deposited on the rear tile, consist of particles having a mean diameter of a few microns and therefore strongly scatter the incident light, that is to say the external light illuminating the panel. This phenomenon causes two types of constraints.
Firstly, in order to have the maximum contrast in the PDP, it is necessary to reduce as much as possible the coefficient of scattered reflection. This coefficient corresponds to the ratio of the intensity of the light reflected by the panel to the intensity of the incident light. However, it is recommended to reduce this coefficient without excessively reducing the luminance emitted by the screen. Since the surface of the panel is not uniformly emissive, a known means of reducing this coefficient consists in blackening the less emissive regions of the panel and thus in forming a “black matrix”. Part of the surface of the panel therefore appears black or dark to an observer, whereas the luminance of the panel is only slightly reduced.
Secondly, in colour display panels, in order to obtain the maximum colour purity, it is recommended to form a screen against the light emitted in the inter-pixel regions. This is because, in these panels, each dot or pixel consists of three cells for the primary colours: red, green and blue. The light discharge in a cell produces ultraviolet light which illuminates the phosphor of a single primary colour which coats the walls of this cell, which phosphor in turn emits this single primary colour. However, this light discharge also illuminates inter-cell regions often having phosphors of various primary colours, which inter-cell regions in turn emit several primary colours; these “parasitic” emissions vary depending on many technological factors and are there not controllable. They degrade the colorimetric purity. The presence of a “black matrix” in the inter-pixel regions allows this degradation to be limited.
In order to understand these phenomena better, the structure of a conventional PDP provided with a “black matrix” will now be described with reference to FIGS. 1 and 2. The PDP in question is of the AC type with surface discharge and separation of the various primary colours by barriers.
The PDP comprises a first glass-tile 2 and a second glass tile 4 a few millimetres in thickness, these being placed and joined together, face to face, with a gap of the order of 100 microns between their internal faces (FIG. 1).
The first tile 2—which constitutes the front face of the PDP—has, on its internal surface, an array of parallel electrodes grouped in pairs of close electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc. Each pair of row electrodes constitutes a display line of the PDP. The electrodes are embedded in a thick layer 6 of dielectric material, for example glass, which coves the entire working area of the tile 2. This layer 6 is itself covered with a thin protective layer 8 (the thickness is less than 1 micron) of another dielectric material, in this case magnesium oxide (MgO), the surface of which is exposed to the discharge gas.
In the example, the internal surface of the first tile 2 is provided with a black matrix 30 which is covered by the dielectric layers 6 and 8 (FIG. 2) and will be explained in detail below.
The second tile 4—which therefore constitutes the rear face of the PDP—has, on its internal face, an array of uniformly spaced parallel electrodes X1, X2, . . . , X6, which are perpendicular to the row electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc., which constitute the array of address electrodes of the plasma display panel.
As in the case of the first tile 2, these electrodes X1, X2 , . . . , X6 are embedded in a thick layer 12 of dielectric which is itself covered with a thin protective layer 14 of magnesium oxide.
A discharge cell of the PDP is thus formed at each intersection between an address electrode X1, X2 , . . . , X6 and a pair of row electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc. of a display line.
In operation, an AC voltage, called sustain voltage, is applied between the electrodes forming the pair of electrodes of each display line. The discharges occur on the surface between these electrodes, depending on a voltage signal applied to the address electrode, using well-established multiplexing techniques.
In particular, it is possible to modify the state of the luminous discharge D (FIG. 2) of each cell by a line-by-line scan in order to produce a display in video mode.
Straight barriers 16 are placed on the thin layer 14 of the second tile 4 between the address electrodes X1, X2, . . . , X6 and parallel to the latter. These barriers 16 have side walls perpendicular to the surface of the tile 4 and a flat top which may possibly serve as a bearing surface for the internal face of the first tile 2. These barriers 16 thus compartmentalize the discharge cells which are located on different address electrodes.
Typically, the barriers 16 have a height of the order of 100 microns and a width of the order of 50 microns and are placed so as to be mutually parallel with a pitch of 220 microns.
Stripes of phosphors 18R, 18G, 18B are placed, between the barriers, on the exposed surface of the second tile 4, more specifically on the thin layer 14 of magnesium oxide. Thus, each stripe of a primary colour, 18R in the case of red, 18G in the case of green and 18B in the case of blue, is bordered by two adjacent barriers 16. Each stripe also covers the side wall of these barriers 16. The phosphors are thus deposited in a repeat pattern of three successive stripes each having a different emission colour, so as to create a succession of elementary colour triads in the direction of the row electrodes X1, X2, . . . , X6.
The two tiles 2 and 4 are sealed together and the space that they enclose is filled with a discharge gas at a low pressure after vacuum-pumping through a pumping tube.
It will be noted that the presence of the layers 6, 8, 12 and 14 of dielectric material on top of the electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc. and X1, X2, . . . , X6 is characteristic of AC PDPs. The dielectric material together with its electrode forms a capacitor across which the voltages needed to generate and sustain the light discharges in the gas are applied.
One specific feature of AC PDPs is that the AC sustain voltage automatically sets the state of a light discharge dot D based on its last command received; either the discharge is sustained or it remains absent, depending on the command previously transmitted. This thus results in an inherent image memory effect, hence the possibility of addressing the dots only when their light state has to change.
The barriers 16 play an important role and determine to a large part the electrooptic characteristics of PDPs, especially with regard to surface-discharge PDPs. This is because they have several separate functions which have a direct impact on the image quality:
they serve as a support for a relatively large part of the phosphor 18 deposited; in this regard, their side walls at right angles to the base of the substrate 2, which are also covered with phosphor, make it possible to obtain a very wide viewing angle; and
since they are opaque, intrinsically or as a result of the phosphor coating, they allow the primary colours to be well separated.
In order to block the parasitic emissions which arise from traces of phosphors deposited on the tops 16 a of the barriers and which degrade the calorimetric purity, a black matrix 30 is deposited on the tile 2 and positioned opposite these tops, i.e. on the front tile, as indicated in FIG. 2. Put another way, stripes of material opaque to visible light are formed—normally on the internal face 2 a of the aforementioned tile—directly opposite the tops 16 a so that an observer looking at the front tile 2 of the panel cannot perceive directly the light emitted by the tops 16 a. The main phases in the production of the front tile 2 provided with a black matrix 30 according to a conventional process will now be described with reference to the flow chart in FIG. 3.
The process starts with a bare glass tile 2′ intended to form the front panel 2 of the PDP. The array of electrodes is deposited (step E2) on this tile in the configuration suitable for this tile. In the example in question, these are row electrodes Y1a-Y1b, Y2a-Y2b , . . . , Y5a-Y5b, etc. The electrodes may be produced by successive layers of conductors, for example as an ITO (indium tin oxide)/chromium-copper-chromium stack or simply a chromium-copper-chromium sequence. A firing step (not shown) may optionally be provided specifically to set these layers.
Next, the black matrix 30 is produced (step E4). This step consists in depositing, on the glass tile 2′ provided with its electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc., a layer of a black dielectric in the pattern required for the matrix. Thus, in the case of the PDP example in FIGS. 1 and 2, the pattern of the black matrix consists of parallel stripes aligned with respect to the tops 16 a of the barriers on the opposite tile.
The black dielectric forms a discontinuous layer of enamel, which is generally composed of pigment particles bound by a glassy matrix (for example, lead borosilicate). The glassy matrix is a mineral substance which serves as sintering and/or binding agent during the firing to vitrify the enamel. The pigment is a mineral substance which, after firing, is sufficiently opaque to visible radiation. It is generally a black pigment. The black matrix 30 is formed before the thick dielectric layer 6 (which is transparent in the front layer) is deposited.
In general, the dielectric forming the black matrix 30 is fired (step E6) before the transparent dielectric 6 is deposited (step E8) so that there is no mixing (interdiffusion) between the black dielectric of the matrix 30 and the transparent dielectric layer 6. After the transparent dielectric layer has been deposited, it is then fired (step E10). Finally, the thin layer 8 of MgO is deposited (step E12) before the equipped and finished tile 2 is obtained.
It may therefore be seen that it is necessary to pass via a firing step E6 especially for producing the black matrix 30, thereby increasing the production costs.
Moreover, the black dielectric made of pigmented enamel has a high optical index, of about 2. Since the refractive index of the glass of the tile is appreciably lower than that of this enamel, there is therefore a change in optical index at the interface between the glass of the tile and the black dielectric 30 which causes a large specular reflection of the incident light. Thus, even if the black matrix 30 is completely absorbent with respect to visible radiation, between 5% and 10% of the incident light illuminating this black matrix would be reflected solely because of the high index of the black dielectric.
BRIEF DESCRIPTION OF THE INVENTION
Given these problems, the present invention provides, according to a first aspect, a composition for a black matrix, especially intended for the production of a plasma display panel, characterized in that it does not include a glassy matrix nor a mineral sintering and/or binding agent.
Advantageously, this composition is in the form of a paste comprising a mixture of at least one pigment and of an organic resin; the nature and the proportions of the resin in the mixture are adapted, in a manner known per se, for allowing this paste to be deposited properly on the tile of the display panel.
The at least one pigment is a mineral product which is opaque to visible light, at least after firing. Preferably, it is a material which is opaque to visible light before firing, which remains temperature-stable up to approximately 600° C., i.e. under any of the conditions for firing the dielectric material usually employed for the manufacture of a plasma display panel.
This pigment is advantageously in the form of particles having a mean size of between 0.1 and 10 microns, and preferably between 0.13 and 5 microns, a typical value being 1.5 microns.
This pigment is preferably chosen from the group comprising: i) an iron chromium aluminium mixed oxide or a mixture of iron, chromium and aluminium oxides, ii) an iron chromium nickel cobalt mixed oxide or a mixture of iron, chromium, nickel and cobalt oxides and iii) an iron chromium cobalt aluminium mixed oxide or a mixture of iron, chromium, cobalt and aluminium oxides.
According to a second aspect, the present invention relates to a plasma display panel characterized in that it comprises a black matrix produced from a composition as described above.
This is especially a plasma display panel comprising a first tile and a second tile facing each other, enclosing a discharge space, and an array of discharge cells at the intersections of electrodes grouped in arrays, each array of electrodes being covered with at least one dielectric and/or protective layer, at least one of the tiles having a black matrix embedded beneath a dielectric or protective layer; according to the invention, the black matrix consists of an opaque material, at least part of which is incorporated into the said dielectric and/or protective layer.
According to a third aspect, the present invention relates to a process for manufacturing a display device, especially a plasma display panel, having a black matrix on a substrate, the process comprising the steps of:
a) producing a substantially opaque layer on the substrate, the said layer being in a pattern corresponding to the black matrix;
b) depositing a dielectric material on the substrate so as to cover the black matrix; and
c) firing the dielectric material.
This process passes from step a) to step b) without an intermediate step of firing the substantially opaque material intended to form the black matrix.
Preferably, before the firing step, the layer making up the black matrix does not include a glassy matrix nor a mineral agent capable of sintering and/or binding during the firing of the dielectric.
Thus, although the “black matrix” is applied according to the invention without a sintering and/or binding mineral agent or a glassy matrix for binding the particles of the black pigment, after the dielectric and/or protective layer which covers this matrix is fired, the pigment particles of the black matrix are partially “wetted” by the glassy phase of the dielectric and/or protective layer. After firing, this partially “wetted” black matrix does indeed form a layer separated from the dielectric layer but it is observed that the enamel of the dielectric layer during the firing has indeed migrated as far as the glass substrate into the intergranular spaces between the pigment particles of the black matrix. Implementation of the process according to the invention makes it possible to avoid a firing step in the manufacture of a tile of a display device and therefore is economically advantageous. By virtue of the only partial wetting of the pigment of the black matrix, the performance of the black matrix is improved, that is to say the specular reflection coefficient is decreased and the colorimetric purity is improved.
The pattern corresponding to the black matrix may be produced by direct screen printing or by photolithography.
If the process is used for manufacturing an AC plasma display panel, it is advantageous to produce the black matrix on the front tile of the panel.